1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting a control signal of a relay station in a wireless communication system.
2. Related Art
Effective transmission/reception methods and utilizations have been proposed for a broadband wireless communication system to maximize efficiency of radio resources. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference.
When a system uses the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology may include a space frequency block code (SFBC), a space time block code (STBC), a cyclic delay diversity (CDD), a frequency switched transmit diversity (FSTD), a time switched transmit diversity (TSTD), a precoding vector switching (PVS), spatial multiplexing (SM) for implementing diversity. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
A user equipment (UE) can transmit an uplink control signal through a physical uplink control channel (PUCCH) assigned by a base station (BS). The uplink control signal may include a hybrid automatic repeat request (HARQ) acknowledgement (ACK)/non-acknowledgement (NACK), a channel quality indicator (CQI) indicating a downlink channel state, a scheduling request (SR) as a request for uplink radio resource assignment, etc.
A wireless communication system employing a relay station (RS) has recently been developed. The RS is employed for cell coverage extension and transmission capability improvement. A BS provides a service to a UE located in a coverage boundary of the BS via the RS, and thus can obtain an effect of extending the cell coverage. In addition, the RS improves signal transmission reliability between the BS and the UE, thereby improving transmission capacity. The RS can be used when the UE is located in a shadow area even if the UE is located within the coverage of the BS.
Similarly to the UE, the RS can also transmit the uplink control signal. The uplink control signal of the RS can be transmitted through a relay R(relay)-PUCCH assigned by using the BS. The R-PUCCH is a channel assigned to transmit the uplink control signal by the RS through a backhaul uplink The uplink control signal of the RS can also include an HARQ ACK/NACK, a CQI, an SR, etc.
Accordingly, there is a need for a method of effectively transmitting a control signal of an RS.